In internal combustion engines, such as diesel engines, warm-up of the engine, particularly under certain climatic conditions, involves an inordinate amount of time. During such warm-up period the various moving components of the engine frequently encounter substantial frictional resistance, requiring the engine to produce more horsepower than would otherwise be the case in order to compensate for such resistance. Thus, in many diesel engines, in order to remedy this situation, a separate heater is utilized to effect rapid heating of the oil and thus, reduce significantly the power loss occasioned during this period in overcoming such frictional resistance. It has also been found with diesel engines that a more efficient operation results if the temperature of the oil remains relatively constant within a predetermined temperature range. To maintain the oil temperature within such a range, it has become a common practice in diesel engine design to employ a separate cooling unit. Thus, to utilize a separate oil heater and a separate cooling unit in an engine is costly, requires additional space to accommodate the engine, and oftentimes such accessories or components involve an inordinate amount of maintenance and servicing.